The popularity of physical training and even sports competitions among middle-age and older adults has increased dramatically since the late 1960's. Though it is generally agreed that such activity has a positive influence on endurance and physical well-being , little information is available to describe the orthopaedic and clinical impact of intense athletic training on the health of this older population. Thus, the intent of the present study will be to examine (1) the effects of 20-30 years of intense training on the rate of decline in aerobic endurance capacity, the influence of endurance training on muscle fiber composition, enzyme activities, strength, and mass in middle aged men, (3) the orthopaedic complications associated with 20-30 years of intense endurance training and competition (4) the incidence of clinical pathology with aging and its relationship to endurance running and (5) the effects of aging and training on the biomechanics of running. This study offers a unique opportunity to reexamine a group of middle-aged men who have continued to train intensely for the past 20-30 years. Three groups of men who originally competed in distance running and were studied in the late 1960's and early 1970's will serve as subject for this investigation. One group of these subjects (TR group) have continued to train for competitive running, whereas the other men now only train for general fitness and occasional competition (RR group) or have ceased to engage in any regular physical activity (UR group). In addition a group of sedentary non-runners will be tested for comparison (UT group). All participants will be examined for orthopaedic and clinical abnormalities, prior to all exercise testing. In addition to resting measurements of blood lipids, electrocardiogram, and muscle biopsies, the men will perform submaximal and a maximal treadmill tests to determine their running economy and aerobic capacities. Strength will be measured in the arms and legs of each subject. X-rays of the knees and nuclear magnetic resonance imaging of the thigh and upper arm will also be performed on subgroups of these men. These observations will provide information relative to both the positive and negative aspects of intense sports training in aging men.